High density semiconductor memory cell and memory array using a single transistor having a buried N+ connection

ABSTRACT

A programmable memory cell comprised of a transistor located at the crosspoint of a column bitline and a row wordline is disclosed. The transistor has its gate formed from the column bitline and its source connected to the row wordline. The memory cell is programmed by applying a voltage potential between the column bitline and the row wordline to produce a programmed n+ region in the substrate underlying the gate of the transistor. Further, the row wordline is formed from a buried N+ layer allowing for higher density integration.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 10/448,505 filed May 30, 2003 entitled “HIGHDENSITY SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING A SINGLETRANSISTOR” and co-pending U.S. patent application Ser. No. 10/133,704filed Apr. 26, 2002 entitled “HIGH DENSITY SEMICONDUCTOR MEMORY CELL ANDMEMORY ARRAY USING A SINGLE TRANSISTOR”, to which priority from both ishereby claimed under 35 USC §120.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to a nonvolatile programmablesemiconductor memory, and more particularly to a single transistormemory cell programmed by break down of the transistor gate oxide, and amemory array incorporating such cells.

BACKGROUND OF THE INVENTION

[0003] Nonvolatile memory retains stored data when power is removed,which is desirable in many different types of electronic devices. Onecommonly available type of nonvolatile memory is the programmableread-only memory (“PROM”), which uses word line-bit line crosspointelements such as fuses, anti-fuses, and trapped charge devices such asthe floating gate avalanche injection metal oxide semiconductor(“FAMOS”) transistor to store logical information.

[0004] An example of one type of PROM cell that uses the breakdown of asilicon dioxide layer in a capacitor to store digital data is disclosedin U.S. Pat. No. 6,215,140 to Reisinger et al. The basic PROM disclosedby Reisinger et al. uses a series combination of an oxide capacitor anda junction diode as the crosspoint element (the term “crosspoint” refersto the intersection of a bitline and a wordline). An intact capacitorrepresents the logic value 0, and an electrically broken-down capacitorrepresents the logic value 1. The thickness of the silicon dioxide layeris adjusted to obtain the desired operation specifications. Silicondioxide has a breakdown charge of about 10 C/cm² (Coulomb/cm²). If avoltage of 10 volts is applied to a capacitor dielectric with athickness of 10 nm (resultant field strength 10 mV/cm), a current ofabout 1 mA/cm² flows. With 10 volts, this thus results in a substantialamount of time for programming a memory cell. However, it is moreadvantageous to design the capacitor dielectric to be thinner, in orderto reduce the high power loss which occurs during electrical breakdown.For example, a memory cell configuration having a capacitor dielectricwith a thickness of 3 to 4 nm can be operated at about 1.5 V. Thecapacitor dielectric does not yet break down at this voltage, so that1.5 V is sufficient to read data from the memory cell. Data are stored,for example, at 5 V, in which case one cell strand in a memory cellconfiguration can be programmed within about 1 millisecond. The energyloss which occurs in this case per cm² of capacitor dielectric is thenabout 50 Watts (10 Coulomb*5 V). If the desired power loss is about 0.5W, about 100 seconds are required to program a 1 Gigabit memory. If thepermissible power losses are higher, the programming can be carried outcorrespondingly more quickly.

[0005] Some types of nonvolatile memory are capable of being repeatedlyprogrammed and erased, including erasable programmable read onlysemiconductor memory generally known as EPROM, and electrically erasableprogrammable read only semiconductor memory generally known as EEPROM.EPROM memory is erased by application of ultraviolet light andprogrammed by application of various voltages, while EEPROM memory isboth erased and programmed by application of various voltages. EPROMsand EEPROMs have suitable structures, generally known as floating gates,that are charged or discharged in accordance with data to be storedthereon. The charge on the floating gate establishes the thresholdvoltage, or V_(T), of the device, which is sensed when the memory isread to determine the data stored therein. Typically, efforts are madeto minimize gate oxide stress in these types of memory cells.

[0006] A device known as a metal nitride oxide silicon (“MNOS”) devicehas a channel located in silicon between a source and drain and overlainby a gate structure that includes a silicon dioxide layer, a siliconnitride layer, and an aluminum layer. The MNOS device is switchablebetween two threshold voltage states V_(TH(high)) and V_(TH(low))byapplying suitable voltage pulses to the gate, which causes electrons tobe trapped in the oxide-nitride gate (V_(TH(high))) or driven out of theoxide-nitride gate (V_(TH(low))). Again, efforts are made to minimizegate oxide stress in these types of memory cells.

[0007] A junction breakdown memory cell that uses a stored charge on thegate of a gate controlled diode to store logic 0 and 1 values isdisclosed in U.S. Pat. No. 4,037,243 to Hoffman et al. Charge is storedon the gate by using a capacitance formed between the p-type electrodeof the gate controlled diode and the gate electrode. Charge storage isenhanced by using a composite dielectric in the capacitor formed fromsilicon dioxide and silicon nitride layers in place of silicon dioxide.The application of an erase voltage to the electrode of the gatecontrolled diode causes the oxide-nitride interface surface to fill withnegative charge, which is retained after the erase operation iscompleted. This negative interface charge causes the gate controlleddiode to operate in an induced junction mode even after the erasevoltage is removed. When the gate controlled diode is thereafter read,it exhibits field-induced junction breakdown of its channel and asaturation current flows. The field induced junction breakdown voltageis less than the metallurgical junction breakdown voltage. However, theapplication of a write voltage to the electrode of the gate controlleddiode causes the silicon dioxide/silicon nitride interface to fill withpositive charge, which is retained after the write operation iscompleted. When the gate controlled diode is thereafter read, it willnot break down because no channel exists. Only a slight current flows.The different current flows are sensed and indicate different logicstates.

[0008] Improvements in the various processes used for fabricating thevarious types of nonvolatile memory tend to lag improvements in widelyused processes such as the advanced CMOS logic process. For example,processes for devices such as flash EEPROM devices tend to use 30% moremask steps than the standard advanced CMOS logic process to produce thevarious special regions and structures required for the high voltagegeneration circuits, the triple well, the floating gate, the ONO layers,and the special source and drain junctions typically found in suchdevices. Accordingly, processes for flash devices tend to be one or twogenerations behind the standard advanced CMOS logic process and about30% more expensive on a cost-per-wafer basis. As another example,processes for antifuses must be suitable for fabricating variousantifuse structures and high voltage circuits, and so also tend to beabout one generation behind the standard advanced CMOS process.

[0009] Generally, great care is taken in the fabrication of the silicondioxide layer used in metal-oxide-silicon (MOS) devices such ascapacitors and transistors. The high degree of care is necessary toensure that the silicon dioxide layer is not stressed during manufactureor subsequent normal operation of the integrated circuit, so that thedesired device characteristics are attained and are stable over time.One example of how much care is taken during fabrication is disclosed inU.S. Pat. No. 5,241,200 to Kuroda, which discloses the use of a diffusedlayer and a shunt to discharge charges accumulated in the word lineduring a wafer fabrication process. Avoiding this charge accumulationensures that a large electric field is not applied to the gateinsulating film, so that variations in the characteristics oftransistors using the word line as their gate wiring line anddegradation and breakdown of the gate insulating film are prevented.

[0010] An example of how much care is taken in circuit design to avoidstressing the silicon dioxide layer of a transistor during normalcircuit operation is disclosed in U.S. Pat. No. 6,249,472 to Tamura etal. Tamura et al. disclose an antifuse circuit having an antifuse inseries with a p-channel MOS transistor in one embodiment and in serieswith an n-channel MOS transistor in another embodiment. While theantifuse is fabricated without the additional film manufacturingprocesses typically required for fabricating antifuse circuits, Tamuraet al. poses another problem. When the antifuse is shorted out, theseries-connected transistor is exposed to a high voltage sufficient tobreak down the silicon dioxide layer of the transistor. Tamura et al.disclose the addition of another transistor to the circuit to avoidexposing the first transistor to the break down potential.

[0011] The observations above generally indicate that there are stilldisadvantages with each of the prior art memory technologies

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic circuit diagram of a portion of a memoryarray in accordance with the present invention.

[0013]FIG. 2 is a partial layout diagram of a portion of the memoryarray represented by FIG. 1.

[0014]FIG. 3 is a cross-section diagram of an integrated circuitstructure for the portion of the memory array corresponding to FIG. 2.

[0015]FIG. 4 is a table of voltages showing the operation of the memorycell of FIGS. 1-3.

[0016]FIG. 5 is a cross-section diagram of a memory cell that has beenprogrammed.

[0017]FIG. 6 is a schematic circuit diagram of a memory cell that hasbeen programmed.

[0018]FIG. 7 is a cross-section diagram of an experimental setup.

[0019]FIG. 8 is a graph showing the effect of a constant voltage stresson an ultra-thin gate oxide.

[0020]FIG. 9 is a graph showing various stages in the current-voltagecharacteristics of an ultra-thin gate oxide as degradation proceeds.

[0021]FIG. 10 is a graph showing time-to-breakdown at 63% distributionvs. gate voltage in a semi-log scale measured on n-channel field effecttransistors (inversion) for various oxide thickness.

[0022]FIG. 11 is a graph showing the current-voltage characteristics ofn-type devices measured after the detection of successive breakdownevents.

[0023]FIG. 12 is a is a partial layout diagram of a portion of thememory array formed in accordance with an alternative embodiment of thepresent invention.

[0024]FIG. 13 is a cross-section diagram of an integrated circuitstructure for the portion of the memory array corresponding to FIG. 12taken along line A-A′.

[0025]FIG. 14 is a cross-section diagram of an integrated circuitstructure for the portion of the memory array corresponding to FIG. 12taken along line B-B′.

[0026]FIG. 15 is a table of voltages showing the operation of the memorycell of FIGS. 12-14.

DETAILED DESCRIPTION

[0027] A semiconductor memory cell having a data storage elementconstructed around a gate oxide is used to store information bystressing the ultra-thin dielectric into breakdown (soft or hardbreakdown) to set a leakage current level of the memory cell. The memorycell is read by sensing the current drawn by the cell. A suitableultra-thin dielectric is the high quality gate oxide of about 10-50 Åthickness or less used in a transistor, as is commonly available frompresently available advanced CMOS logic processes. Such oxides arecommonly formed by deposition, by oxide growth from a silicon activeregion, or by some combination thereof. Other suitable dielectricsinclude oxide-nitride-oxide composites, compound oxides, and so forth.

[0028] In the following description, numerous specific details areprovided to provide a thorough understanding of embodiments of theinvention. One skilled in the relevant art will recognize, however, thatthe invention can be practiced without one or more of the specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures, materials, or operations are not shownor described in detail to avoid obscuring aspects of the invention.

[0029] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0030] The present invention is related to other types of gate oxidebreakdown based non-volatile memory designs developed by the presentinventor and assigned to the same assignee as the present invention.Examples are shown in U.S. patent application Ser. No. 09/955,641 filedon Sep. 18, 2001 entitled “SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAYUSING A BREAKDOWN PHENOMENA IN AN ULTRA-THIN DIELECTRIC”, U.S. patentapplication Ser. No. 10/024,327 filed on Dec. 17, 2001 entitled“SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING A BREAKDOWN PHENOMENAIN AN ULTRA-THIN DIELECTRIC”, U.S. patent application Ser. No.09/982,034 filed on Oct. 17, 2001 entitled “SMART CARD HAVINGNON-VOLATILE MEMORY FORMED FROM LOGIC PROCESS”, and U.S. patentapplication Ser. No. 09/982,314 filed on Oct. 17, 2001 entitled“REPROGRAMMABLE NON-VOLATILE OXIDE MEMORY FORMED FROM LOGIC PROCESS”,each of which is hereby incorporated by reference. However, in each ofthe memory cells described above, the cell size is relatively large. Thepresent invention provides a much smaller cell size, thereby allowing ahigher density.

[0031]FIG. 1 shows an example of a memory array 100 formed in accordanceof with the present invention. The memory array 100 is a four column bythree row array, however, it can be appreciated that the array can beany arbitrary size. The memory array 100 includes twelve memory cells102, each of which comprises a MOS transistor 104. The memory cell 102at, for example, the crosspoint of the first row R₁ and the first columnC₁ includes a MOS transistor 104 having its gate connected to a columnline C₁ (also referred to herein as a “bitline” or “column bitline”),its source connected to a row line R₁ (also referred to herein as a“wordline” or “row wordline”), and its drain left floating connected toa drain of an adjacent memory cell 102. Alternatively, as will be seenbelow, since there is no current through the drains, the drains ofadjacent devices need not be connected where a shallow trench isolation(STI) is used to isolate two memory cells

[0032] As will be seen below, during the programming step, a relativelylarge voltage is applied to the gate of the transistor 102 (through thebitline C_(x), where x=1 to M and M is the total number of columns) ofthe selected column to break down the gate oxide of the transistor 102.The other memory cells 102 shown in FIG. 1 are also formed from, in oneembodiment, identical transistors 102 at the crosspoints of the columnbitlines C_(x) and row wordlines R_(y), where y=1 to N and N is thetotal number of columns.

[0033] The use of transistors 102 as data storage elements in the memoryarray 100 of FIG. 1 is advantageous because the transistors can befabricated using many conventional CMOS processes using only a singlepolysilicon deposition step, without adding any mask steps to them. Thisis in contrast to “floating gate” type flash memory, which requires atleast two polysilicon layers. Further, with modern technologicaladvances, the size of a transistor can be made very small. For example,current 0.18 micron, 0.13 micron, and smaller line width processes wouldgreatly increase the density of the flash memory.

[0034] Although only a four by three memory array 100 is shown, inpractice such memory arrays contain on the order of about one gigabit ormore of memory cells when fabricated using, for example, an advanced0.13 μm CMOS logic process. Even larger memories will be realized asCMOS logic processes improve further. The memory array 100 in practiceis organized into bytes and pages and redundant rows (not shown), whichmay be done in any desired manner. Many suitable memory organizationsare well known in the art.

[0035]FIG. 2 shows a partial layout diagram 200 for a portion of thememory array 100, and FIG. 3 presents a cross-section of an illustrativeMOS integrated circuit 300 showing the principal structural aspectsthereof corresponding to the memory cells 102 formed by transistors 104in accordance with the layout diagram of FIG. 2. The layout diagram ofFIG. 2 is suitable for an advanced CMOS logic process. The term MOS iscommonly understood to pertain to any gate material, including dopedpolysilicon and other good conductors, as well as to various differenttypes of gate dielectrics not limited to silicon dioxide, and the termis so used herein. For example, the dielectric may be any type ofdielectric, such as an oxide or nitride, which undergoes a hard or softbreakdown upon the application of a voltage for a period of time. In oneembodiment, a thermally grown gate silicon oxide of about 50 angstroms(50 A for 0.25 um, 30 A for 0.18 um, 20 A for 0.13 um, 16 A for 0.09 umprocess) thick is used.

[0036] The memory array 100 preferably is laid out in a grid in whichthe column lines such as C₁, C₂, C₃, and C₄ are orthogonal to the rowlines such as R₁, R₂, and R₃, as well as the diffused source and drainsof the transistors 104. The transistor 104 at the crosspoint of the rowline R₁ and the column line C₁ is formed in a p-well active region 302in the following manner.

[0037] An ultra-thin gate oxide layer 304 is formed by either depositionor thermal oxidation. This is followed by a deposition and doping of apolysilicon layer, which is patterned using a gate mask containingpatterns for the column bitlines C₁, C₂, C₃, and C₄, which also serve asgates 310 for the transistors 104. Alternatively, the column bitlinesmay be separate structure that are connected to the gates 310 of thetransistors through column bitline segments. The various source anddrain regions are formed by conventional process steps (implants,spacers, and n+ source/drain implants), creating the n+ source regions306 and n+drain regions 308. Importantly, it should be noted that thepolysilicon gate 310 for the transistors 104 should not overlap with then+ source/drain regions. Thus, a lightly doped drain structure is notemployed. As will be seen below, by not having the polysilicon gate 310overlap or proximal to the n+ source/drain regions, during programming,the polysilicon gate will not short directly to the n+ source/drainregions.

[0038] Further, contacts (also referred to as row wordline segments) ton+ source regions 306 are formed to allow connection with the row linesR_(y). The row lines R_(y) are formed from a metal deposition that issubsequently etched. Further, an interlayer dielectric (not shown) isdeposited over the polysilicon layer. Thus, the contact vias connectingthe metal row lines R_(y) to the n+ source regions 306 are formed withinthe interlayer dielectric.

[0039] The operation of the memory array 100 is now explained withreference to the illustrative voltages shown in FIG. 4. It will beappreciated that the voltages are illustrative, and that differentvoltages are likely to be used in different applications or whendifferent process technologies are used. During programming, the variousmemory cells in the memory array 100 are exposed to one of four possibleprogramming voltage combinations, which are shown on lines 401, 403,405, and 407 of FIG. 4. Read voltages are shown on lines 409, 411, 413,and 415. Assume that a memory cell 102 is selected for programming andis located at the crosspoint of R₁ and C₁. The selected memory cell 102is referred to as at the selected row and selected column (“SR/SC”). Asshown on line 401, the voltage on the selected wordline R₁ (designatedas V_(wl or “voltage on the wordline”) is) 0 volts and the voltage onthe bitline C₁ (designated as V_(bl) or “voltage on the bitline”) is aprogramming voltage (V_(pp)), in this case 8 volts. Thus, the voltageacross the gate (the bitline C₁) of the transistor 104 and the source(the wordline R₁) of the transistor 104 is 8 volts. The gate oxide 304of the transistor 104 is designed to break down at this potentialdifference, which programs the memory cell. During programming, thevoltage potential breaks down the gate oxide and results in a leakagecurrent through the gate oxide into the underlying substrate andcollected mostly by the N+ source/drain, which is connected to ground.Further, the result of this is that a programmed n+ region 501 (see FIG.5) is formed in the p-well 302 between the n+ source region 306 and then+ drain region 308 of the transistor 104.

[0040] It can be appreciated that the precise magnitude of voltagesapplied is dependent upon the thickness of the gate oxide and otherfactors. Thus, for example, for a 0.13 micron CMOS process, the gateoxide is typically thinner, thereby requiring a lower voltagedifferential between the selected wordline and the selected bitline. Inone embodiment, where a 0.13 micron CMOS process is used, the bitlineC₁, and the unselected word line has a voltage of 4.5 volts and theunselected bitline R₁ has a voltage of between 0 and 1.2 volts.

[0041] With R₁ and C₁ being the selected row and column, consider theimpact on the memory cell 102 at the crosspoint of a selected row and anunselected column (“SR/UC”), e.g., R₁ and C₂. As shown on line 405, thevoltage on the wordline R₁ is 0 volts and the voltage on the unselectedbitline C₂ is 0 or floating. This causes a relatively low potentialdifference across the gate oxide 304 of the transistor 104, which isinsufficient to break down the gate oxide for the transistor 104 at thecrosspoint. The memory cell 102 does not program under these conditions.

[0042] With R₁ and C₁ being the selected row and column, consider theimpact on the memory cell 102 at the crosspoint of a selected column andan unselected row (“UR/SC”), e.g. R₂ and C₁. As shown on line 403, thevoltage on the unselected wordline R₂ is floating or V_(pp) and thevoltage on the bit line C₁ is at V_(pp) (8 volts in this example). Thiscauses a relatively low potential difference across the gate oxide 304of the transistor 104. The memory cell 102 does not program under theseconditions.

[0043] With R₁ and C₁ being the selected row and column, consider theimpact on the memory cell 102 at the crosspoint of an unselected columnand an unselected row (“UR/UC”), e.g. R₂ and C₂. As shown on line 407,the voltage on the unselected wordline R₂ is floating or V_(pp) and thevoltage on the unselected bitline C₂ is 0 volts or floating. This causesa negative potential difference across the gate 304 and the N+source/drain of the transistor 104. Since the N+ source/drain ispositive and gate is negative, the higher voltage on the source/drainwill not pass under the gate, so the memory cell 102 does not programunder these conditions. Further, the voltage on the unselected wordlinecould be biased to an intermediate voltage, such as 2V to 6V to preventthe cell from being programmed. However, the programmed cell will causea leakage current from the selected bitline to the unselected wordline.If the unselected bit line is floating, the leakage current will chargeit up, which causes the voltage to raise up in the bitline. By biasingthe unselected wordlines R_(x) to V_(pp), we can prevent this leakageand thus the charge time of the selected bitline through programmedcells can be reduced.

[0044] After a memory cell 102 has been programmed by breaking down thegate oxide 304, the physical characteristics of the cell 102 is changed.Turning to FIG. 5, a transistor 104 of the memory cell 102 has beenprogrammed. During programming, a programmed n+ region 501 is formedunderneath the gate of the transistor 104. This programmed n+region 501is formed as current (during the programming process) pierces throughthe gate oxide 304 and deposits into the substrate (p-well 302).

[0045] Although difficult to view clearly in FIG. 3, as noted above, thepolysilicon gate 310 for the transistors 104 should not overlapvertically with the n+ source/drain regions 306 and 308. Indeed, theseparation laterally between the gate 310 and the n+ source region 306and n+ drain region 308, by for example the use of a CMOS LDD spacer,should be sufficient to prevent short circuiting during programming. Asseen in FIG. 3, this lateral separation is designated as a lateraldistance D. In one embodiment, the lateral distance D is between 0.02microns to 0.08 microns as formatted by a LDD dielectric spacer in CMOSlogic devices. By not having the polysilicon gate overlap or proximal tothe n+ source/drain regions, during programming, the polysilicon gatewill not short directly to the n+ source/drain regions. Instead, theprogrammed n+ region 501 is formed. Further, alternative methods bywhich short circuiting is avoided between the gate 310 and the n+regions 306 and 308 may be utilized. As but one example, the gate oxidenear the n+ regions 306 and 308 may be made thicker by poly gatesidewall oxidation after the gate poly etch. It can be appreciated thatother methods are also suitable.

[0046] The programmed memory cell of FIG. 5 can be seen in schematicform in FIG. 6. The result of programming a memory cell is the formationof two gated diodes 601 and 603. The gated diodes 601 and 603 preventcurrent from flowing from the wordlines R_(y) to the bitlines C_(x).However, current will be allowed to flow from the bitlines C_(x) to thewordlines R_(y) during the read operation since the positive gate biascan induce a n+ inversion, which can make the connection to the N+source/drain regions.

[0047] The memory array 100 is read in the following manner. A readselect voltage V_(RD) (for example 1.8 volts) is placed on the selectedcolumn bitline (“SC”) and a read select voltage of 0 volts is placed onthe selected row wordline (“SR”). Note that these voltages are for atypical 0.18 micron CMOS process. Lower voltages would be typically usedfor smaller more advanced CMOS processes. For example, for a 0.13 micronCMOS process, the read select voltage on the selected column bitline maybe approximately 1.2 volts.

[0048] Assume that R₁ and C₁ are the selected row and column (“SC/SR”)and that the memory cell 102 at that crosspoint is programmed. As shownon line 409, 1.8 volts (a read select voltage) is applied via bitline C₁to the gate of the transistor 104 and 0 volts is applied to the sourcevia the wordline R₁. This causes a current to flow from the bitline C₁,through the gate oxide of the transistor 104, and out through thewordline R₁, which is ground to zero. By detecting the current on thebitline, it can be determined whether the memory cell 102 is programmed.If the memory cell 102 is not programmed, no current will flow, whichindicates that the memory cell is not programmed.

[0049] With R₁ and C₁ being the selected row and column for the readoperation, consider the impact on the memory cell 102 at the crosspointof a selected column and an unselected row (“UR/SC”), e.g. R₂ and C₁. Asshown on line 411, 1.8 volts is on the selected bitline C₁ and thesource via the unselected wordline R₂ is held to floating or V_(RD).There is no voltage potential across the transistor and no current flow,which indicates that the memory cell is not programmed. By biasing theunselected wordline R₂ to V_(RD), the charge time of the selectedbitline through programmed cells can be reduced. This is because if theunselected wordline is floating, it will take some time to charge it upto by the selected bit through the programmed cell.

[0050] With R₁ and C₁ being the selected row and column for the readoperation, consider the impact on the memory cell 102 at the crosspointof an unselected column and a selected row (“SR/UC”), e.g. R₁ and C₂. Asshown on line 413, 0 volts is on the unselected bitline C₂ and 0 voltsis applied to the source via the selected wordline R₁. There is novoltage potential across the transistor and no current flow, whichindicates that the memory cell is not programmed.

[0051] With R₁ and C₁ being the selected row and column for the readoperation, consider the impact on the memory cell 102 at the crosspointof an unselected column and an unselected row (“UR/UC”), e.g. R₂ and C₂.As shown on line 415, 0 volts is on the unselected bitline C₂ and thesource via the unselected wordline R₂ is held to floating or V_(RD).Even for the cell that was previously programmed, and the programmedcell acts like a reversed biased diode, so there is no current goingfrom the unselected wordline (1.8V) to the unselected bitline (0V),which indicates that the memory cell is not programmed.

[0052] Thus, as seen above, during the read cycle, no current is drawnby memory cells at crosspoints having either an unselected row or anunselected column. Note that the unselected wordlines may be leftfloating. This embodiment will tend to reduce leakage current throughthe wordlines, as well as allowing the use of smaller word line drivers,thereby saving integrated circuit space.

[0053] Moreover, in an alternative embodiment, in order to increase then+ source/drain junction breakdown voltage and reduce junction leakagein the case of an unselected wordline charged to V_(pp)^(either from the selected word line through prior programmed cells or through a word line driver, a high energy, low dose n+ implant can be used. The implant may be a standard n+ electrostatic discharge protection implant from a conventional CMOS process or other existing implant steps, thus staying within standard CMOS logic process. Still, in other embodiments, a special implant step may be added to optimize the implanting.)

[0054] An alternative embodiment of the present invention is shown inFIGS. 12-14. The operation table of this alternative embodiment is shownin FIG. 15. In the alternative embodiment of FIG. 12, the row wordlinesR₁ and R₂ are not formed from a metal deposition as is shown in theembodiment of FIG. 2. Instead, the row wordlines R₁ and R₂, and ingeneral all of the row wordlines R_(Y), are formed from a buried n+layer formed in the substrate. The buried n+ layer thus replaces themetal wordlines discussed above. Because of this, there is no need formetal contact to connect the row wordlines R_(Y) to the N+ sourceregions 306. In general, this allows for higher density integration ofthe memory array.

[0055] For clarity purposes, it should be noted that the buried N+ layerforming the row wordlines R₁ and R₂ of FIG. 12 is shown, while the N+source regions 306 and the n+ drain regions 308 are not shown in the topview of FIG. 12.

[0056]FIG. 13 is a cross-section view of the silicon substrate takenalong line A-A′ of FIG. 12. The buried N+ layer 1301 is formed justunderneath the N+ source regions 306 and N+drain regions 308. Indeed,the N+ source regions 306 are in electrical contact with the buried N+layer 1301. Thus, the buried n+ layer 1301 replaces the metal row lineR_(y) of FIG. 2. Additionally, the n+ drain regions 308 are also incontact with the buried N+ layer 1301.

[0057]FIG. 14 shows a cross-section view of the substrate taken alongine B-B′ of FIG. 12. In this embodiment, shallow trench isolations(“STI”) 1401 serve to separate and isolate the memory cells. The buriedn+ layer 1301 is shown under the surface of the substrate, but stillseparated by the shallow trench isolations 1401.

[0058] The formation of the buried n+ layers 1301 will requireadditional masking and implantation steps. In one embodiment, or arsenicmay be used as the dopent instead of phosphorous in order to limit thediffusion layer thickness in the deep submicron process. The buried n+layer 1301 can be formed using high energy ion implantation eitherbefore or after the formation of the thin gate oxide layer and/or thepolysilicon deposition. Alternatively, the buried n+ layer 1301 may bedeposited using an epitaxial deposition technique. Further, in order tobe CMOS logic process compatible, the lightly doped P-type implant isthe same as the logic NMOS threshold voltage V_(t) implant.

[0059] Compared to the embodiment shown in FIG. 2, the buried n+ layer1301 can reduce the size of the memory array by 50% or more due to thesmaller critical dimensions possible with the photolithography stepsrelative to metal and contact via formulation.

[0060] Finally, FIG. 15 shows the operation table for the embodiment ofFIGS. 12-14. The programming voltage V_(pp) is approximately 8-9 voltsfor a gate oxide thickness of 32 angstroms or 5-6 volts for gate oxidethickness of 20 angstroms. Typically, the V_(DD) is the input/outputvoltage and is on the order of 3.3 volts or 2.5 volts. The power supplyvoltage of V_(CC) is typically 1.8 volts for 0.18 micron process or 1.2volts per 0.13 micron process. As you can see, there can be a range ofvoltages used in order to form the programming and reading functions.Note also that at lines 405 and 407 (for the unselected columns), thevoltage on the column bitline V_(bl) is less than 0.5 volts. If theunselected column bitline is greater than 0.5 volts (in other wordsV_(t)), the previously programmed cells along the common column bitlinewill have a large leakage current through the programmed cells. Bylimiting V_(bl) to under V_(t), this leakage current can be reduced oreliminated.

[0061] Various studies of oxide breakdown, which were performed incontexts different than the memory cells 102 shown in the array 100,indicate suitable voltage levels for breaking down ultra-thin gateoxides and establishing that the breakdown is controllable. When anultra-thin gate oxide is exposed to voltage-induced stress, breakdown inthe gate oxide occurs. Although the actual mechanisms leading to theintrinsic breakdown of gate oxide are not well understood, the breakdownprocess is a progressive process passing through a soft breakdown(“SBD”) stage followed by a hard breakdown (“HBD”) stage. One cause ofbreakdown is believed to be oxide defect sites. These may act alone tocause breakdown, or may trap charges and thereby cause high local fieldsand currents and a positive feedback condition that leads to thermalrunaway. Improved fabrication processes resulting in fewer oxide defectsare reducing the occurrence of this type of breakdown. Another cause ofbreakdown is believed to be electron and hole trapping at various siteseven in defect-free oxide, which also leads to thermal runaway.

[0062] Rasras et al. performed a carrier separation experiment whichdemonstrated that, under positive gate bias, impact ionization of theelectrons in the substrate is the dominant source of the substrate holecurrent. Mahmoud Rasras, Ingrid De Wolf, Guido Groeseneken, RobinDegraeve, Herman e. Maes, Substrate Hole Current Origin after OxideBreakdown, IEDM 00-537, 2000. A constant voltage stress experiment wasperformed on ultra-thin oxide in an arrangement in which channelinversion was involved, and established that both SBD and HBD may beused for storing data, and that a desired degree of SBD or HBD may beobtained by controlling the time over which the gate oxide storageelement is stressed. FIG. 7 shows a schematic cross-sectionalrepresentation of the experimental setup. The effect of the constantvoltage stress on the ultra-thin gate oxide is shown in the graph ofFIG. 8, in which the x-axis is time in seconds and the y-axis is currentin amperes expressed logarithmically. FIG. 8 shows the gate andsubstrate hole current measured before and after soft and hard breakdownunder constant voltage stress. For roughly 12.5 seconds, the totalcurrent is substantially constant and dominated by an electron currentas measured by I_(g). The leakage is believed to be due toFowler-Nordheim (“FN”) tunneling and stress-induced leakage current(“SILC”). At about 12.5 seconds, a large jump in the measured substratehole current is observed, which signals the onset of a soft breakdown(“SBD”). The total current remains substantially constant at this newlevel, albeit with some fluctuation in the substrate current, from about12.5 seconds to about 19 seconds. At about 19 seconds, large jumps inboth the electron current and the substrate hole current signal theonset of hard breakdown (“HBD”). FIG. 8 shows that a desired degree ofSBD or HBD may be obtained by controlling the time over which the gateoxide storage element is stressed.

[0063] Sune et al. studied post SBD conduction in ultra-thin silicondioxide films. Jordi Sune, Enrique Miranda, Post Soft Breakdownconduction in SiO2 Gate Oxides, IEDM 00-533,2000. Various stages in thecurrent-voltage (“I-V”) characteristics of an ultra-thin gate oxide asdegradation proceeds are shown in FIG. 9, in which the x-axis is voltagein volts and the y-axis is current in amperes expressed logarithmically.FIG. 9 shows that a broad range of voltages may be used to program thegate oxide storage element, and that either SBD or HBD may be used tostore information in the gate oxide storage element. Several postbreakdown I-V characteristics are also included that show the evolutionfrom SBD to HBD. The amount of the leakage current resulting at SBD andHBD as well as at the intermediate situations between these two extremesis roughly linearly dependent on the magnitude of the voltage in a rangeof about 2.5 volts to 6 volts.

[0064] Wu et al. studied the voltage dependency of voltage accelerationfor ultra-thin oxides. E. Y. Wu et al., Voltage-DependentVoltage-Acceleration of Oxide Breakdown for Ultra-Thin Oxides, IEDM00-541, 2000. FIG. 10 is a graph of time-to-breakdown at 63%distribution vs. gate voltage in a semi-log scale measured n channelFETs (inversion) for oxide thickness varying from 2.3 nm to 5.0 nm. Thedistributions are in general agreement and are linear, furtherindicating that the process is controllable.

[0065] Miranda et al. measured the I-V characteristics of nMOSFETdevices having an oxide thickness of 3 nm and an area of 6.4×10⁻⁵ cm²after the detection of successive breakdown events. Miranda et al.,“Analytic Modeling of Leakage Current Through Multiple Breakdown Pathsin SiO₂ Films”, IEEE 39th Annual International Reliability PhysicsSymposium, Orlando, Fla., 2001, pp 367-379. FIG. 11 shows the resultscorresponding to the linear regime in which “N” is the number ofconducting channels. The results are quite linear, indicating that thepath is essentially resistive.

[0066] The transistor used in the memory cells described herein in mostcases are normal low voltage logic transistors having, for example, anultra-thin gate oxide thickness on the order of 50 Å for a 0.25 μmprocess, or on the order of 20 Å for a 0.13 μm process. The voltageacross such an ultra-thin gate oxide can be temporarily duringprogramming much higher than V_(CC), which typically is 2.5 volts for anintegrated circuit fabricated with a 0.25 μm process, and 1.2 volts foran integrated circuit fabricated with a 0.13 μm process. Such ultra-thinoxides typically can stand up to as much as 4 or 5 volts withoutsignificant degradation on the transistor performance.

[0067] The description of the invention and its applications as setforth herein is illustrative and is not intended to limit the scope ofthe invention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments are known to those of ordinaryskill in the art. For example, the various voltages set forth in thevarious examples are only illustrative, since one has some discretion asto the precise voltage to select within a range of voltages, and thevoltages are in any event dependent on the device characteristics. Theterms row wordline and column bitline have been used to describe typesof lines commonly used in memories, but some memories may havealternatives thereto. Further, the various doping types may be reversed,such that an n-channel transistor described above may be replaced with ap-channel transistor. In such a situation, the p-channel transistorwould be formed in a large n-well and a buried p+ layer may be used.These and other variations and modifications of the embodimentsdisclosed herein may be made without departing from the scope and spiritof the invention.

1. A programmable memory cell useful in a memory array having columnbitlines and row wordlines, the memory cell comprising: a transistorhaving a gate, a gate dielectric between the gate and over a substrate,and first and second doped semiconductor regions formed in saidsubstrate adjacent said gate and in a spaced apart relationship todefine a channel region therebetween and under said gate, the gate beingformed from one of said column bitlines; and wherein the second dopedsemiconductor region of the transistor is connected to one of said rowwordlines, wherein said row wordlines are formed from a buried N+ layer.2. The memory cell of claim 1 wherein the gate dielectric of thetransistor is thicker proximal to the first and second dopedsemiconductor regions than at said channel region.
 3. The memory cell ofclaim 1 wherein the gate and said second doped semiconductor region islaterally separated by a distance D.
 4. The memory cell of claim 3wherein said distance D is sufficient to prevent a short circuit withsaid first or second doped semiconductor regions.
 5. A method ofoperating a programmable memory array comprising a plurality of rowwordlines, a plurality of column bitlines, and a plurality of memorycells at respective crosspoints of the row lines and column lines, saidmemory cells comprising a transistor having a gate, a gate dielectricbetween the gate and over a substrate, and first and second dopedsemiconductor regions formed in said substrate adjacent said gate and ina spaced apart relationship to define a channel region therebetween andunder said gate, the gate being formed from one of said column bitlines,and the second doped semiconductor region of the transistor connected toone of said row wordlines, said row wordlines being formed from a buriedn+ layer, the method comprising: applying a first voltage to a selectedone of the column bitlines and gate of a selected transistor; andapplying a second voltage to a selected one of the row wordlines;wherein the first voltage and the second voltage form a potentialdifference across the gate dielectric of said selected transistor tocause the formation of a programmed doped region in said substrate insaid channel region of said selected transistor.
 6. The method of claim5 wherein said selected transistor is read by applying a fourth voltageon the gate of said selected transistor and monitoring for a currentflowing from said gate to said selected column bitline.
 7. Aprogrammable memory array comprising a plurality of row wordlines, aplurality of column bitlines, and a plurality of memory cells atrespective crosspoints of the row wordlines and column bitlines, each ofthe memory cells comprising: a transistor having a gate, a gatedielectric between the gate and over a substrate, and first and seconddoped semiconductor regions formed in said substrate adjacent said gateand in a spaced apart relationship to define a channel regiontherebetween and under said gate, the gate being formed from one of saidcolumn bitlines; and wherein the second doped semiconductor region ofthe transistor is connected to one of said row wordlines, wherein saidrow wordlines are formed from a buried N+ layer.
 8. The memory array ofclaim 7 wherein said column bitlines are connected to said gate by acolumn bitline segment.
 9. The memory array of claim 7 wherein the gatesof said transistors do not overlap either of said first and second n+doped semiconductor regions of said respective transistor.
 10. Thememory array of claim 8 wherein the gate dielectric of the transistorsare thicker proximal to the respective first and second dopedsemiconductor regions than at said channel region.
 11. The memory arrayof claim 8 wherein said transistors have their gate and said seconddoped semiconductor region is laterally separated by a distance D. 12.The memory array of claim 11 wherein said distance D is sufficient toprevent a short circuit from the gate to said first or second dopedsemiconductor regions.
 13. The memory array of claim 18 wherein saidmemory cells further including a programmed doped region formed in saidsubstrate in said channel region when said memory cell has beenprogrammed.
 14. A programmable memory cell useful in a memory arrayhaving column bitlines and row wordlines, the memory cell comprising: atransistor having a gate, a gate dielectric between the gate and over asubstrate, and first and second doped semiconductor regions formed insaid substrate adjacent said gate and in a spaced apart relationship todefine a channel region therebetween and under said gate, the gate beingformed from one of said column bitlines; and wherein the second dopedsemiconductor region of the transistor is connected to one of said rowwordlines, wherein said row wordlines are formed from a buried P+ layer.15. The memory cell of claim 14 wherein the gate dielectric of thetransistor is thicker proximal to the first and second dopedsemiconductor regions than at said channel region.
 16. The memory cellof claim 14 wherein the gate and said second doped semiconductor regionis laterally separated by a distance D.
 17. The memory cell of claim 16wherein said distance D is sufficient to prevent a short circuit withsaid first or second doped semiconductor regions.
 18. A method ofoperating a programmable memory array comprising a plurality of rowwordlines, a plurality of column bitlines, and a plurality of memorycells at respective crosspoints of the row lines and column lines, saidmemory cells comprising a transistor having a gate, a gate dielectricbetween the gate and over a substrate, and first and second dopedsemiconductor regions formed in said substrate adjacent said gate and ina spaced apart relationship to define a channel region therebetween andunder said gate, the gate being formed from one of said column bitlines,and the second doped semiconductor region of the transistor connected toone of said row wordlines, said row wordlines being formed from a buriedp+ layer, the method comprising: applying a first voltage to a selectedone of the column bitlines and gate of a selected transistor; andapplying a second voltage to a selected one of the row wordlines;wherein the first voltage and the second voltage form a potentialdifference across the gate dielectric of said selected transistor tocause the formation of a programmed doped region in said substrate insaid channel region of said selected transistor.
 19. The method of claim18 wherein said selected transistor is read by applying a fourth voltageon the gate of said selected transistor and monitoring for a currentflowing from said gate to said selected column bitline.